From conventional technology, potential changes on the surface of a solid body due to chemical reactions are known. For example, these potential changes may be measured using a so-called “Kelvin probe”, wherein a plate capacitor having a mechanically moved electrode is used. Due to the mechanical movement a charge transfer results which is detected via voltage (difference of the surface potentials of the two electrodes or “contact potential difference” (CPD)). If a chemical reaction changes the charge on an electrode surface, the resulting change of the CPD, i.e. the change of the electric field, may be measured. To acquire reliable measurements it is desirable for one of the two electrodes not to be chemically reactive, i.e. to be inert, e.g. in the sense of a reference electrode. Otherwise, a reaction of the chemically sensitive electrode might not be determined correctly.
On the basis of the Kelvin probe there have been numerous developments. In order to avoid using the mechanically moved electrode, field effect transistors (FETs) have been used, wherein a potential change leads to a transfer or shift of the initial voltage and thus to a current change. FETs having a metal insulator semiconductor structure turned out to be especially suitable, see also (MISFETs). The metal insulator semiconductor structure may here form a capacity between a metal layer and a semiconductor layer. By the MIS capacitor, with the help of a voltage at the metal (also “gate”) a charge may be influenced in the semiconductor. The same controls the current between two electrodes (also “source/drain”).
For the application of MISFETs as gas or liquid sensors, an air gap between the metal and the insulator is realized and a chemically sensitive layer is applied on the side of the metal facing the insulator. Embodiments are of the suspended gate FET (SGFET), the hybrid SGFET (HSGFET) and the charge coupled FET (CCFET). The MIS capacity may be reduced by the additional air gap, and consequently also the current control factor for the transistor and thus also the sensitivity regarding chemically-generated potential changes. An improvement may be reached by using an insulated gate, also known as “floating gate” (FGFET). Here a center tap is introduced into an MIS capacitor having an air gap, which is connected to a conventional metal oxide layer FET (MOSFET) as a “floating”/insulated electrode.
Although the chemically generated potential changes at the center tap decreases due to the capacitive voltage division, all in all an improvement of the sensitivity may result as the amplification factor of the MOSFET may be fully utilized. With this concept, the gate electrode of the MIS capacitor is provided with a chemically sensitive layer which may be problematic. The opposing electrode is covered by an insulator. The air gap is located between this insulator and the gate electrode. The open insulator surface may be problematic as it may also react chemically, e.g. due to moisture, corrosion, etc.
Apart from that, a free surface has a finite resistance which may lead to drift phenomena due to the electrical fields within the capacitor.
For being applied as a liquid sensor, the metal electrode (the gate of the MISFET) is replaced by a conductive liquid (ISFET). If the liquid is provided with a reference electrode, at the insulator/liquid interface a space charge region forms within the liquid whose width depends on the ion concentration. The thus resulting capacity change changes the current control factor of the transistor and thus enables the measurement of the ion concentration in the liquid. Also here, a stable reference electrode in the liquid is of importance.